docs: def add(self, statisticType, onStatisticField, outStatisticFieldName=None):
          """
          Adds the statistics group to the filter.
  
          outStatistics - is supported on only those layers/tables that
            indicate supportsStatistics is true.
          outStatisticFieldName is empty or missing, the map server assigns a
            field name to the returned statistic field. A valid field name
            can only contain alphanumeric characters and an underscore.
          outStatisticFieldName is a reserved keyword of the underlying DBMS,
            the operation can fail. Try specifying an alternative
            outStatisticFieldName. When using outStatistics, the only other
            parameters that can be used are groupByFieldsForStatistics,
            orderByFields, time, and where.
          """
          val = {
              "statisticType" : statisticType,
              "onStatisticField" : onStatisticField,
              "outStatisticFieldName" : outStatisticFieldName
          }
          if outStatisticFieldName is None:
              del val['outStatisticFieldName']
          self._array.append(val)